Ten beach cruisers is on the order of 300 lb, plus another 100 lb for the racks. That's comparable to two adult passengers plus some luggage. So if it's just you and the bikes (and perhaps one passenger), you're not exceeding the load the car's drivetrain and suspension was designed to handle. The roof rack is a little worse aerodynamically than a trailer, but the trailer adds more weight. Neither will add any undue stress on the drivetrain if you keep your foot out of the throttle.

I'd go with the trailer. Loading several heavy bikes on a roof rack is not really something I'd want to do even a couple times a year (let alone 8). A towed trailer would make loading and unloading a lot easier and would also allow you to carry a full complement of passengers and/or gear inside the car without any worry about overloading the suspension/tires. The trailer also has the advantage of being useful for hauling a wide variety of non-bicycle stuff.